Work machines such as wheel type loaders include work implements capable of being moved through a number of positions during a work cycle. Such implements typically include attachments such as buckets, forks, and other material handling apparatus which are coupled to lift arm, or boom, movably connected to the work machine via lingages. The typical work cycle associated with a bucket includes sequentially positioning the bucket and boom in a digging position for filling the bucket with material, a carrying position, a raised position, and a dumping position for removing material from the bucket. To protect the boom against the implement or linkages being “slammed” into it, the boom is provided with a plurality of rack and dump stops placed on the respective upper and lower surfaces of the boom. Each rack and dump stop is typically strategically sized and arranged to engage a corresponding portion of either the attachment, the attachment linkages, or both, thereby concentrating any attachment impact to selected areas of the boom. In addition, rack and/or dump stops are typically attached, by use of mechanical fasteners, to the attachment.
Control levers are mounted at the operator's station and are connected to an electrohydraulic circuit for moving the bucket and/or boom. The operator must manually move the control levers to open and close electrohydraulic valves that direct pressurized fluid to hydraulic cylinders which in turn cause the implement to move. For example, when the boom is to be raised, the operator moves the control lever associated with the boom hydraulic circuit to a position at which a hydraulic valve causes pressurized fluid to flow to the head end of a lift cylinder, thus causing the boom to rise. When the control lever returns to a neutral position, the hydraulic valve closes and pressurized fluid no longer flows to the lift cylinder.
Under certain operating conditions, the attachment or linkage may make contact with the boom. For example, when the attachment is placed in the dump cycle, the attachment may contact the under portion of the boom as the operator attempts to either dislodge material from, or load material into, the attachment. Likewise, contact between the attachment or linkage and the top portion of the boom may occur when the operator attempts to “catch” or cause material to be caught by the attachment. If not properly inspected and maintained, missing or damaged rack and dump stops can lead to excessive forces placed on the boom. These forces may damage the boom, as well as damage the associated hydraulic circuitry that absorb some of the shock that travels through the linkage assembly. This will likely increase maintenance and accelerated failure of the associated parts.
To reduce the forces acting upon the work implement, systems have been developed to more slowly and smoothly stop the motion of the implement. One such system is disclosed in U.S. Pat. No. 5,617,723 issued to Hosseini et al. on Apr. 8, 1997. A method is provided which uses joystick and implement position sensors for controlling a sudden change in inertia of a work implement of a work machine. While this system adequately reduces the velocity of the work implement during sudden changes in operator control settings, it is not operable to control the movement of a work implement in response to missing rack or dump stops.
An alternate system is disclosed in U.S. Pat. No. 5,511,458, issued to Kamata et al. on Apr. 30, 1996. This system utilizes cylinder position and movement direction detectors to provide a quiet cylinder cushioning effect. Although this system may also be adequate for its intended purpose, it also is not operable to control the movement of a work implement in response to missing rack or dump stops.
The present invention is directed to overcoming one or more of the problems as set forth above.